1. Field of the Invention
The present invention relates to an image processing technique, and particularly to a technique to prevent, in a case where a monochrome image is generated based on a color image, chromatic image information on the color image from being lost.
2. Description of the Related Art
Hitherto, as CCD line sensors used in a reduction optical system, there are known a line sensor including a 1-line line sensor, and a 3-line line sensor in which color filters of red (hereinafter denoted by R), green (hereinafter denoted by G) and blue (hereinafter denoted by B) are respectively arranged on surfaces of three line sensors.
In the case of the structure using the 1-line CCD line sensor, it is basically used for monochrome original document reading. When a color original document is read by using this CCD line sensor, there is adopted a method in which three light sources having spectral characteristics of R, G and B as three primary colors of light are successively turned on, and image information of the color original document is divided into color information of R, G and B and is read. Besides, there is also known a structure in which a light source having a spectral characteristic of white light is used, color filters of R, G and B are arranged in a light path between this light source and the line sensor, and color information incident on the line sensor is separated by switching the color filters.
On the other hand, in the case of the structure using the 3-line CCD line sensor, it is basically used for color original document reading. The light source in this case has a spectral characteristic to sufficiently cover the visible light region of oscillation wavelengths of 400 nm to 700 nm, and the separation of color information of R, G and B is performed by color filters arranged on surfaces of the respective line sensors.
Besides, in the case where a monochrome original document is read by using the 3-line CCD sensor, there are a case of using one output among the three line sensors, in general, the line sensor output of G for the purpose of certainly reading a vermilion seal, and a case of using all the three line sensors to generate a monochrome image signal.
In the case where a color original document is read by a general monochrome scanner using a line sensor in which a color filter is not arranged on a light receiving surface, since reflected light from the original document is incident on the line sensor, the change in luminance can be read, however, information relating to colors can not be read. Thus, for example, in the case where information is formed of a red letter on the blue ground of the original document, although a spectral characteristic of a light source has also an influence, in the case where the reflectivity is the same, blue and red are indistinguishable and are processed as the same monochrome signal, and there has been a problem that the information existing on the color original document is lost.
Besides, in the case where the color original document is copied in monochrome by using the 3-line line sensor in which the color filters of red (hereinafter denoted by R), green (hereinafter denoted by G) and blue (hereinafter denoted by B) are respectively disposed on the surfaces of the three line sensors, according to the color of the sheet of the color original document, the same color occurs, and there is a case where information on the color original document is lost.
In a scanner, since reflected light from an original document is focused on the respective line sensors and image information is read, color information is reproduced by an additive color process of red, blue and green as the three primary colors of light.
Besides, there is also known a system in which an achromatic color is pseudo-generated by adding wavelength regions of red, blue and green of the color filters on the line sensors and averaging them. In this case, the calculation can be performed as follows: monochrome information (gray scale information)=(red information+blue information+green information)/3.
However, when the average value as stated above is used, for example, in the case of the original document in which information is described by a red letter on a blue sheet (ground), and in the case where the outputs of the respective line sensors at the time of reading the information of the blue ground are (red:blue:green)=(0:255:0), and the outputs of the respective line sensors at the time of reading the information of the red letter are (red:blue:green)=(255:0:0), when the blue ground information is made monochrome, (0+255+0)/3=85 is obtained, and when the red letter information is made monochrome, (255+0+0)/3=85 is obtained. That is, the red letter information described on the blue sheet is lost.
Similarly, even if images are different from one another in the balance (chromaticity) of red, blue and green, since all information in which the additive result of red, blue and green is the same is treated as the same information and as signals of monochrome copying, in the case where the color original document is copied in monochrome, there has been a case where a letter or an image is lost.